Magnesium: Physicochemical Properties, Minerals and Chemical Reactions

Physicochemical Properties

Magnesium has three stable isotopes: 24Mg, 25Mg, and 26Mg. All are present in significant amounts. About 79% of Mg is 24Mg, while 25Mg covers about 10% and 26Mg the remaining 11%. The isotope 28Mg is radioactive and in the 1950s to 1970s was manufactured in several nuclear power plants for application in scientific research. This isotope has a relatively short half-life (21 hours), and its application was restricted by shipping times. The nuclide 26Mg has found application in isotopic geology, like that of aluminum. 26Mg is a radiogenic daughter product of 26Al, which has a halflife of 717,000 years. Excessive amounts of 26Mg have been found in the Ca-Al-rich inclusions of some carbonaceous chondrite meteorites.

Mg is a light but tough silvery-white metal and the second member of the alkaline earth metals with the electronic configuration of [Ne]3s2 (Table 4.2). It crystallizes in an hcp arrangement, which confers a marked anisotropy in its properties. The predominant 12 oxidation state of magnesium and other group 2 metals can be attributed to their electronic configuration, ionization energies, and size.

Minerals

Magnesium is the eight-most-abundant element in the earth’s crust by mass and tied in seventh place with Fe in molarity. It is found in large deposits of magnesite, MgCO3, dolomite, CaMg(CO3)2, and other minerals, and in mineral waters, where magnesium ion is soluble.

Magnesium is a common element in many minerals with more than 800 minerals containing this element, either on its own or in combination with similar metals in terms of charge and size such as Fe and Mn substituting for each other on the same position in the crystal structure. Within the sulfide group, only a handful minerals contain Mg, for example, niningerite ((Mg,Fe2+,Mn2+)S), oldhamite ((Ca,Mg)S), and tochilinite (Fe2+ 5- 6(Mg; Fe²⁺)₅S₆(OH)10). The halides contain 18 minerals with Mg, such as carnallite (KMgCl3· 6H2O), chloromagnesite (MgCl2), and sellaite (MgF2). More than 50 oxide minerals contain Mg in their structure. Here one finds minerals such as brucite (Mg(OH)2) (Fig. 1), magne-siochromite (MgCr2O4), periclase (MgO), and spinel (MgAl2O4) (Fig. 2). In the carbonate class, 41 minerals can be found that contain Mg, for example, artinite (Mg2(CO3)(OH)2· 3H2O), dolomite (CaMg(CO3)2) (Box 3), hydrotalcite (Mg6Al2(CO3)(OH)16· 4H2O), magnesite (MgCO3) (Fig. 4), pyroaurite Mg6Fe³⁺₂ (OH)16 [CO3]· 4H2O , and stichtite Mg6Cr31 2 OH ð Þ16 [CO3  ]· 4H2O  . Over 50 borate minerals contain Mg, for example, boracite (Mg3(B7O13)Cl), harkerite (Ca12Mg4Al(BO3)3(SiO4)4(CO3)5· H2O), inderite (MgB3O3(OH)5· 5H2O), kurnakovite (MgB3O3(OH)5· 5H2O), ludwigite (Mg2Fe31(BO3)O2), and sinhalite (MgAl(BO4)). About the same number of sulfate minerals exist that contain Mg, for example, botryogen (MgFe3+(SO4)2(OH)· 7H2O), epsomite (MgSO4· 7H2O), hexahydrite (MgSO4· 6H2O), magnesiocopiapite MgFe3+ 4 SO4 ð Þ6 OH ð Þ2· 20H2O, pickeringite (MgAl2(SO4)4· 22H2O), and polyhalite (K2Ca2Mg(SO4)4· 2H2O). The phosphate class has more than 100 minerals that have Mg in their structure. Here one finds minerals such as collinsite (Ca2Mg(PO4)2· 2H2O), gordonite (MgAl2(PO4)2(OH)2· 8H2O), lazulite (MgAl2(PO4)2(OH)2), montgomeryite (Ca4MgAl4(PO4)6(OH)4· 12H2O), sale´eite (Mg(UO2)2(PO4)2· 10H2O), and struvite ((NH4)Mg(PO4)· 6H2O). The largest group of Mg-containing minerals can be found in the silicate class with more than 250 minerals, for example, actinolite ({Ca2}{Mg4.5-2.5Fe0.5-2.5}(Si8O22)(OH)2), antigorite/chrysotile/ lizardite (Mg3(Si2O5)(OH)4), augite ((CaxMgyFez)(Mgy1Fez1)Si2O6), cordierite ((Mg,Fe)2Al3(AlSi5O18)), diopside (CaMgSi2O6) (Fig. 5), dravite (Na(Mg3)Al6(Si6O18)(BO3)3(OH)3(OH)), enstatite (MgSiO3), forsterite (Mg2SiO4), phlogopite (KMg3(AlSi3O10)(OH)2), pyrope (Mg3Al2(SiO4)3), talc (Mg3Si4O10(OH)2), tremolite ({Ca2}{Mg5}(Si8O22)(OH)2), vermiculite (Mg0.7(Mg,Fe,Al) 6 (Si,Al)8O20(OH)4· 8H2O), and vesuvianite ((Ca, Na,) 19 (Al, Mg, Fe31) 13 ( B, Al, Fe31) 5 (Si2O7)4(SiO4) 10 (OH, F, O)10).

Chemical Reactions

Magnesium reacts with water at room temperature forming bubbles of hydrogen gas and magnesium hydroxide. Increasing the temperature speeds up this reaction.

Magnesium tarnishes in air, forming an oxide layer around itself to prevent it from further oxidation. However, when ignited in air and especially when it is in strips or powder and in pure oxygen, it burns with a brilliant white light. The flame from burning magnesium can reach up to 3100℃ and is not easily extinguished once started. This is because the combustion continues with nitrogen at temperatures around 800℃ and form magnesium nitride; with carbon dioxide to form magnesium oxide (MgO) and carbon; and with water to form magnesium hydroxide and hydrogen. The flammability of magnesium is reduced by a small amount of calcium in magnesium alloys.

Magnesium is more electropositive than the amphoteric Be above it and reacts more readily with most of the nonmetals. It ignites with the halogens, particularly when they are moist, to give MgX2. For example, when Mg is heated in the presence of chlorine gas, it ignites to form magnesium chloride (MgCl2).

Magnesium readily dissolves in acids and forms solutions that have both the Mg21 ions and hydrogen gas. When reacted with bases, magnesium form basic hydroxide solutions.

Organic compounds of magnesium are commonly in the form of Grignard reagents. Grignard reagents are formed by the reaction of magnesium metal with alkyl of alkenyl halides following a single electron transfer mechanism as shown below.

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